Field of the Invention
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine.
Description of the Related Art
It is known that in an exhaust passage of an internal combustion engine which is able to be operated by an air fuel mixture of a lean air fuel ratio, there are arranged a three-way catalyst, an NOx storage reduction catalyst (hereinafter referred to as an NSR catalyst) and an NOx selective catalytic reduction catalyst (hereinafter also referred to as an SCR catalyst) sequentially in this order from an upstream side. The NSR catalyst serves to occlude or store NOx contained in an incoming exhaust gas when the oxygen concentration of the incoming exhaust gas is high, and to reduce the occluded or stored NOx when the oxygen concentration of the incoming exhaust gas becomes low and when a reducing agent exists. Because there is a limit in the amount of NOx able to be stored in the NSR catalyst, when a certain amount of NOx is stored in the NSR catalyst, rich spike control is carried out in order to recover the NOx storage ability of the NSR catalyst. The rich spike control is processing to temporarily control the air fuel ratio of the exhaust gas to a rich air fuel ratio. By carrying out this rich spike control, NOx is released from the NSR catalyst, and the NOx thus released is reduced by a reducing agent such as HC, so that the storage amount of NOx in the NSR catalyst is decreased.
On the other hand, the SCR catalyst serves to selectively reduce NOx in the exhaust gas by using ammonia as a reducing agent. Then, ammonia is produced by the reaction of HC or H2 in the exhaust gas with NOx in the three-way catalyst or the NSR catalyst. This ammonia can be used as the reducing agent in the SCR catalyst. In the above-mentioned catalyst arrangement, when the rich spike control is carried out, ammonia may be produced in the three-way catalyst or the NSR catalyst.
Here, there is also known a technology in which a target value of ammonia to be adsorbed by the SCR catalyst is calculated, and only in cases where an integrated value of the amounts of ammonia produced in the three-way catalyst and the NSR catalyst is less than the target value, ammonia is produced (for example, refer to a first patent literature).